The immunologic interplay of the intact spleen with other lymphoid organs during the rejection or acceptance of organ allografts or the induction of graft-versus-host disease (GVHD) is not yet fully understood. Preliminary studies have shown that vascularized whole-organ spleen allografts exchanged between inbred strains of guinea pigs, incompatible at the major histocompatibility complex (MHC), survived indefinitely in the absence of immunosuppression in immunocompetent hosts. The underlying mechanisms of these phenomena are still unknown and are the subject of the proposed research.